Vfrdtyky

How to swap Armadillo objects, e.g. arma::vecs, without swapping contents?

void f5()

{

  arma::vec x(10);

  arma::vec y(10);

  std::cout << &x[2] << ", " << &y[2] << "n";

  x.swap(y);

  std::cout << &x[2] << ", " << &y[2];

}

The above code outputs

0x24fbe50, 0x24fbef0

0x24fbe50, 0x24fbef0

Thank you!

Looks like armadillo's swap is internally a memcpy below a certain array size (according to op <=16).

A small expansion of @OZ17's answer.
Armadillo seems to store data with sizes< 16 locally mem_localand larger ones in an area pointed out by mem

From GDB:

> p x

{

  <arma::Mat<double>> = {

    <arma::Base<double, arma::Mat<double> >> = {

    <arma::Base_inv_yes<arma::Mat<double> >> = {<No data fields>}, 

    <arma::Base_eval_Mat<double, arma::Mat<double> >> = {<No data fields>}, 

    <arma::Base_trans_default<arma::Mat<double> >> = {<No data fields>}, <No data fields>}, 

    members of arma::Mat<double>: 

    n_rows = 1, 

    n_cols = 10, 

    n_elem = 10, 

    vec_state = 2, 

    mem_state = 0, 

    mem = 0x7fffffffb830, 

    mem_local = {0 <repeats 16 times>},   

    static is_col = false, 

    static is_row = false

  }, 

  members of arma::Row<double>: 

  static is_col = false, 

  static is_row = false

}

and a small example to visualize it:

arma::rowvec x(10,arma::fill::ones);

arma::rowvec y(10,arma::fill::zeros);

std::cout << "Size=10" << std::endl;

std::cout << "&x=" << x.memptr() << ", x[0..4]=" << x.subvec(1,5);

std::cout << "&y=" << y.memptr() << ", y[0..4]=" << y.subvec(1,5);

x.swap(y);

std::cout << "x.swap(y)" << std::endl;

std::cout << "&x=" << x.memptr() << ", x[0..4]=" << x.subvec(1,5);

std::cout << "&y=" << y.memptr() << ", y[0..4]=" << y.subvec(1,5);



arma::rowvec x2(17,arma::fill::ones);

arma::rowvec y2(17,arma::fill::zeros);

std::cout << "nSize=17" << std::endl;

std::cout << "&x=" << x2.memptr() << ", x[0..4]=" << x2.subvec(1,5);

std::cout << "&y=" << y2.memptr() << ", y[0..4]=" << y2.subvec(1,5);

x2.swap(y2);

std::cout << "x.swap(y)" << std::endl;

std::cout << "&x=" << x2.memptr() << ", x[0..4]=" << x2.subvec(1,5);

std::cout << "&y=" << y2.memptr() << ", y[0..4]=" << y2.subvec(1,5);

The output from the example shows that the content is swapped in both cases but for small arrays it has swapped the local mem area and for the larger case it has swapped the mem pointer.

Size=10

&x=0x7fffffffb830, x[0..4]=   1.0000   1.0000   1.0000   1.0000   1.0000

&y=0x7fffffffb8e0, y[0..4]=        0        0        0        0        0

x.swap(y)

&x=0x7fffffffb830, x[0..4]=        0        0        0        0        0

&y=0x7fffffffb8e0, y[0..4]=   1.0000   1.0000   1.0000   1.0000   1.0000



Size=17

&x=0x5555557d7fd0, x[0..4]=   1.0000   1.0000   1.0000   1.0000   1.0000

&y=0x5555557d8060, y[0..4]=        0        0        0        0        0

x.swap(y)

&x=0x5555557d8060, x[0..4]=        0        0        0        0        0

&y=0x5555557d7fd0, y[0..4]=   1.0000   1.0000   1.0000   1.0000   1.0000